The technology described herein relates to a magnetometer for medical use, such as for use as a cardiac magnetometer.
It can be useful in many medical situations to be able to measure magnetic fields relating to or produced by the human body. For example, magnetic field measurements can be useful for diagnosing and investigating bladder conditions, foetal abnormalities, pre-term labour, and the heart, and for encephalography.
It is known, for example, that measurements of the magnetic field of the heart can provide useful information, for example for diagnostic purposes. For example, the heart's magnetic field contains information that is not contained in an ECG (Electro-cardiogram), and so a magneto-cardiogram scan can provide different and additional diagnostic information to a conventional ECG.
Modern cardiac magnetometers are built using ultra-sensitive SQUID (Superconducting Quantum Interference Device) sensors having a noise floor between 1-1000 fT/√Hz. Such devices perform well and have a sound-diagnostic capability.
However, SQUID magnetometers are very expensive to operate as they require cryogenic cooling. Their associated apparatus and vacuum chambers are also bulky pieces of equipment. This limits the suitability of SQUID magnetometers for use in a medical environment, for example because of cost and portability considerations.
Another known form of magnetometer is an induction coil magnetometer. Induction coil magnetometers have the advantage over SQUID magnetometers that cryogenic cooling is not required, they are relatively inexpensive and easy to manufacture, they can be put to a wide range of applications and they have no DC sensitivity.
However, induction coil magnetometers have not been adopted for magneto-cardiography. This is because magneto-cardiography requires low field (<nT), low frequency (<100 Hz) sensing, and existing induction coil magnetometer designs that can achieve such sensitivities are too large to be practical for use as a cardiac probe.
For example, when looking to design a very sensitive induction coil magnetometer, the conventional approach would be to try to maximise the inductance of the coil. The Brooks coil (which is defined, for example, in: Grover, F. W.; Inductance calculations, working formulas and tables; 1946: D. Van Nostrand) is a well-known design of induction coil that maximises the inductance for a given length of wire. The Brooks coil recognises and teaches that the optimum value of the induction will be obtained with a coil having a square winding cross-section with the sides of the square equal to the radius of the core. FIG. 1 illustrates this and shows the configuration of a Brooks coil: a square winding cross-section having a diameter (a side-length) a, with the core radius also being a. However, a coil of this configuration that has the sensitivity needed for magneto-cardiography will then have a diameter that is too large to provide the spatial resolution that is needed for magneto-cardiography.
Thus the literature currently teaches away from using induction coil magnetometers for magnetocardiography, notwithstanding their apparent advantages over SQUID sensors, as it is not believed possible to achieve a sufficiently sensitive induction coil magnetometer whilst still achieving sufficient spatial resolution to be medically useful.
The Applicants believe therefore that there remains scope for improvements to the design and use of magnetometers for medical use, and in particular for cardio-magnetic imaging. In particular, a compact, portable and relatively inexpensive device that can image magnetic fields of the human body, such as the magnetic field of the heart, would provide a number of significant advantages over existing medical and cardiomagnetometer designs.
Like reference numerals are used for like components where appropriate in the Figures.